Agreed on both counts!
Seriously, with car jacking these days, stopping because someone in an unmarked car wants you to, and even if they 'appear' to be wearing a police uniform, is that wise? With no other audible or visual signs that it is police that is risky and if the situation left me with no option I would stay in the car with doors locked and windows shut and have the phone primed to ring 999 on speaker if necessary.
Might sound a bit drastic but remember we drive in cars that (so called) joy riders would love to get their hands on if they could.
I would only stop if I saw blue lights personally, as in the link if someone was 'chasing' you or trying to force you off the road then drive to a heavily populated place